1. Field of the Invention
This invention relates to a defect inspection method and to a defect inspection device for a magnetic recording medium.
2. Description of the Related Art
In recent years, hard disk drives have tended toward larger capacities, and recording densities of a magnetic recording medium have continued to rise. Conventional longitudinal recording methods are methods in which magnetization is parallel to a medium in-plane direction; in a magnetic recording medium employing this method, when recording densities are high magnetization reversals due to thermal fluctuations exert a considerable influence, so that a limit to high recording densities had been approached. On the other hand, in perpendicular magnetic recording methods, in which the magnetization is perpendicular to the medium plane, as recording densities are raised the demagnetizing field within the recording magnetization tends to stabilize the magnetization, and thermal fluctuations due to the demagnetizing field do not readily occur. Hence in recent years a perpendicular magnetic recording medium has been considered for application in high-density recording.
However, increases in track density accompanying higher recording densities result in the problem of interference with adjacent tracks. Hence a discrete-track type magnetic recording medium has been proposed, in which grooves are provided between recording tracks of the magnetic recording medium. In a discrete-track type magnetic recording medium, intervals between tracks are filled with a nonmagnetic material, so that interference between tracks can be reduced (for example, see Japanese Patent Application Laid-open No. H9-97419).
Further, in a recording medium known as a bit patterned medium, in addition to providing grooves between tracks, recording elements corresponding to single bits can be made independent by also providing grooves between bits. Thus, interference between adjacent tracks as well as interference between adjacent bits can be reduced. A recording medium having such patterns in the medium surface are called a patterned medium (and will hereafter be called a patterned medium); in addition to grooves demarcating tracks or bits, servo data used in positioning control in a hard disk device is also provided as a pattern on the medium surface. The patterned medium also includes a medium in which only a servo pattern is provided on the medium surface.
A hard disk device causes a head to be lifted at approximately 10 nm above a hard disk, and performs data recording and reproduction. If there is a defect in the surface of the hard disk, then serious problems such as head flight fluctuations and head damage may occur. Hence it is necessary to perform tests to determine whether a magnetic disk satisfies performance conditions deemed the minimum necessary prior to incorporation in a hard disk drive (HDD). Such tests are generally performed using a test head, to inspect the magnetic disk glide characteristics and certification characteristics. In glide characteristic inspections, the number of protrusions on the magnetic disk are determined. In certification characteristic inspections, the presence or absence of electric characteristics and defects of the magnetic disk is confirmed. In the test of certification characteristics, specific signals are recorded on the magnetic recording medium, and the level of the reproduced signals is compared with a reference signal level to inspect for the presence of defects in the magnetic disk.
For example, as a perpendicular magnetic recording medium defect inspection method, a dropout detection method, in which output fluctuations in the reproduced signals from a perpendicular magnetic recording medium which has been DC-demagnetized in advance are compared with a reference signal, is known (see for example Japanese Patent Application Laid-open No. 2004-199733).
However, in methods of the prior art, certification characteristic inspections of magnetic disks using discrete track designs must be performed with a servomechanism applied. This is because, as shown in FIG. 1A, the situation is satisfactory when the inspection head 11 is positioned above a track 13, but when the inspection head 11 traverses a groove 12 under the influence of eccentricity, as in FIG. 1B. (that is, when the inspection head 11 deviates from the track 13), it is substantially impossible to reproduce signals.
On the other hand, in defect inspections of an ordinary (grooveless) magnetic disk medium, no servomechanism is applied, and so defect inspection devices used in current mass production processes often do not have servo functions. Hence in order to perform inspections with a servomechanism applied, tasks must be performed to improve the stage portion which moves the head to a VCM (Voice Coil Motor) similar to that used in hard disk devices.
A method has been disclosed in which a magnetic head is used having a broad width compared with the width of tracks on the magnetic disk to inspect the magnetic disk, and performing defect inspections using signals obtained by cutting out the low-frequency components of reproduced signals with a filter (see for example Japanese Patent Application Laid-open No. 2002-15420).
In certification inspections of so-called a discrete track medium and patterned medium, having patterns in the medium surface corresponding to servo data and other preformat information, the servo region and other preformat regions must be masked to perform defect inspections in data regions.
Normally, servo signals include preamble signals used to generate a servo clock, SAM (Servo Address Mark) signals indicating a servo region, address signals indicating the track number and sector number, and burst signals used in precise positioning. In addition, signal-free regions are provided between servo regions and data regions. To detect servo signals in a patterned medium, first a signal-free region is detected, and then a servo clock is generated in synchronization with the preamble signal, and the servo clock thus generated is used to detect the signal that follows. If the detect signal matches a SAM signal, then the region is identified as a servo region, and the address signal following the SAM signal is detected. Thereafter the burst signal is detected.
To detect servo regions, this servo demodulation must be performed, and a device which performs certification inspection must include a servo data demodulator. However, servo data generally differs among products, and it is difficult to incorporate a servo data demodulator that supports different products.